I have a 1991 Dodge power ram 150 4x4 short bed truck with an A-518 non lock up transmission. I had the transmission re-built in September of 2012. Paid a lot of money for the re-build too. The transmission shop gave me a 2 year warranty on it which is expired now. When I asked about what fluid to run they said dexron3 and when I asked about a synthetic they said stay away from them the dexron3 will be just fine. So I have been running Napa house brand dexron3 in it and haven't had any issues it still shifts well when it's cold or hot. I'm going to be having some upgraded parts installed in this transmission, ether this spring with my tax return or this coming summer, I am going to have some 6 pinion steel planeterys installed up front in place of the aluminum 4 pinion planeterys and I am going to have a trans go shift kit installed. Transmission has to come out for these upgrades.m While its out I have decided I might as well run a better fluid then dexron3, it makes no sense to me to run such a dated fluid in a transmission that I will eventually have over 3 thousand dollars invested in.. I have been thinking about trying ether amsoil universal atf or mobil 1 synthetic universal or red line d4 atf or maybe even royal purple atf. I have also considered running the new dexron6 because I have heard it is a very good atf as well. People have also recommended valvoline maxlife to me. What synthetic fluid would work best forme? I live in idaho where winters are brutally cold and summers get pretty warm to, winter can get as cold as 30 below zero summer can get as hot as 100 degree. I have an external transmission cooler along with the factory tranny cooler.